Head-mounted and other compact display systems, such as head-up displays (HUDs), laptops, monitors, and other systems often rely on optical devices that overlay images onto transparent optical elements. For example, systems may display images through transparent lenses in front of a user's eyes to enable the user to view displayed images at the same time as viewing objects in the environment. Some of these display systems may utilize substrate-guided relays having three primary components: an input coupler, a transmission substrate, and an output coupler. Light beams or other electromagnetic waves enter the relay via the input coupler and remain confined within the relay due to total internal reflection or applied coatings. The beams are guided to the output coupler within the substrate, and the beams exit the relay via the output coupler.
Various techniques may be used to insert light into substrate-guided relays. Some current systems use diffractive collimation (where LCD panels transmit light to a holographic element that inserts light into a relay) and/or refractive collimation (where lenses insert LCD produced light into the relay). However, these systems require increases in size in order to expand the field of view or exit pupil expansion. Display systems relying on these techniques are therefore large and heavy when a wide field of view is desired.
Various techniques may be used to couple light out of substrate-guided relays. Some current relays employ transmission substrates that have mirrors embedded in the substrate to couple light from the substrate. Although relays relying on this structure may provide a desirable form factor, the relays require numerous reflectors each having a different and precise reflectance characteristic, which create discontinuities in an image within the relay and images seen through the relay. They often do not efficiently couple light out of the substrate.
A short-coming of existing substrate-guided relays is that they are not compatible with recent advances in light source technology. For example, current substrate-guided relays are sub-optimal when used with scanned beam light sources to create images, such as systems that use MEMS based scanning mirrors to scan a beam across two axes to create an image. Scanned beam light sources produce narrower input beams and current relays have poor efficiency and do not generate sufficient duplicates to produce a uniform image or pupil for smaller scanned beam based images. Additional problems arise when expanding the field of view or the pupil with current relays, making the relays difficult to fabricate and manufacture.
These and other problems exist with adapting current substrate-guided relays for scanned beam light sources.